1. Field of the Invention
The present invention relates to a aluminophosphate molecular sieve and to its synthesis. More particularly, the present invention relates to the synthesis of a class of aluminophosphate molecular sieves using sulfonium salts as directing agents in the synthesis. The particular class of aluminophosphate molecular sieves to which the present process is directed may also include components such as comprising aluminum, phosphorus, and an optional metal other than aluminum which is incorporated into the crystalline structure.
2. Background of the Invention
Much research effort continues to be expended on discovering new molecular sieves, and on new methods of preparing molecular sieves. In conventional usage the term "molecular sieve" refers to a material having a fixed, open-network structure, usually crystalline, that may be used to separate hydrocarbons or other mixtures by selective occlusion of one or more of the constituents, or may be used as a catalyst in a catalytic conversion process. The term "zeolite" refers to a molecular sieve containing a silicate lattice, usually in association with some aluminum, boron, gallium, iron, and/or titanium. In the following discussion and throughout this disclosure, the terms molecular sieve and zeolite will be used more or less interchangeably. One skilled in the art will recognize that the teachings relating to zeolites are also applicable to the more general class of materials called molecular sieves.
These sieves have been used in a number of important commercial processes. For example, U.S. Pat. No. 4,913,799 discloses the use of non-zeolitic molecular sieves, including metalloaluminophosphates, for hydrocracking processes. U.S. Pat. No. 4,859,314 discusses catalytic cracking processes using similar molecular sieves. U.S. Pat. No. 4,859,311 discloses a dewaxing process using a SAPO-11 type molecular sieve. Considering the benefit which has been derived from the use of known sieves, this work will continue for many years to come.
In the preparation of a molecular sieve, a reaction mixture comprising active sources of the molecular sieve is maintained at crystallization conditions until crystals are formed. Often, the crystallization process is aided by templates or structure directing agents which are included in the reaction/crystallization medium. In the conventional preparation, the templating agent is typically an organic cation which contains nitrogen or phosphorus. Although desirable, and in some cases critical, the use of templating agents for directing the crystallization process may increase the cost and complexity of manufacturing, and may pose a handling and disposal problem after crystallization is complete. Thus, there continues to be a need for new, effective templating agents at reduced manufacturing cost and at reduced environmental and disposal hazard.
An aluminophosphate molecular sieve is any one of a number of crystal structures comprising [AlO.sub.2 ] and [PO.sub.2 ] tetrahedra. Adding an additional metal into the crystalline framework increases the number of possible crystalline forms. As described above, the formation of aluminophosphate molecular sieve crystals may be enhanced by adding to the reaction mixture prior to crystallization an effective amount of a templating agent. For example, U.S. Pat. No. 4,310,440 teaches microporous aluminum phosphate materials with uniform pores and having a chemical composition, expressed in terms of molar ratios of oxides, of EQU Al.sub.2 O.sub.3 :1.0.+-.0.2P.sub.2 O.sub.5.
The templating agents taught in U.S. Pat. No. 4,310,440 include organic amines or quaternary ammonium salts.
Silicoaluminophosphates, as taught in U.S. Pat. No. 4,440,871, for example, are microporous, crystalline materials having a three-dimensional crystalline framework of PO.sub.2.sup.+, AlO.sub.2.sup.-, and SiO.sub.2 tetrahedral units. U.S. Pat. No. 4,440,871 teaches preparing silicoaluminophosphates using templating agents disclosed for use in the synthesis of conventional zeolite aluminosilicates and microporous aluminophosphates. The proposed structure directing agents in U.S. Pat. No. 4,440,871 include compounds containing nitrogen, phosphorus, arsenic and antimony.
U.S. Pat. No. 4,567,029 recites crystalline metal-containing aluminophosphates having aluminum, phosphorus and element "M" incorporated into the three-dimensional microporous framework structure, where "M" represents at least one metal selected from magnesium, manganese, zinc and cobalt. Preferred structure directing agents in the '029 reference contain elements of Group VA of the Periodic Table of Elements, particularly nitrogen, phosphorus, arsenic, and antimony. U.S. Pat. No. 4,913,795 and U.S. Pat. No. 4,913,796 teach using C.sub.5 -C.sub.7 alkyldiamines for making metalloaluminophosphates.
Several patents disclose using sulfur-containing templating agents for preparing aluminosilicate zeolites. U.S. Pat. No. 4,557,917 teaches using an anionic template containing a carboxylic acid (preferably a sulfonated succinic acid) moiety for the preparation of ZSM-5. U.S. Pat. No. 4,639,360 teaches using an anionic organosulfonic acid structure directing agent for preparing ZSM-5.
However, none of these patents suggest the use of cationic organosulfonium templating agents for the preparation of aluminophosphate molecular sieves. In particular, the anionic sulfur-containing organic materials identified above as having utility as templating agents in the preparation of molecular sieves have very different chemical action when compared with the organosulfonium structure directing materials of the present invention.